An electronic control unit (ECU) is configured so that various electronic elements are arranged on a printed-wiring board. The electronic element is an electronic part utilizing conduction of electrons. The ECU and electrical equipment disposed at the outside of the ECU and connected to the ECU form an electrical and electronic system.
In the electrical and electronic system, the electrical equipment is disposed at the outside of the ECU, and thus electro static discharge (ESD) such as high-voltage static charge or the like may enter from the external side to the electrical equipment. In this case, the electronic elements in the ECU may be broken by the high-voltage ESD. Therefore, various countermeasures to protect the electronic elements have been implemented to the electrical and electronic system.
For example, as disclosed in JP-A-3-49524, a capacitor conducted to the ground (GND) is connected to a line through which the outside and inside of the ECU are connected to each other. In this case, ESD entering from the external side can be made to leak through the capacitor to the ground. Accordingly, the electrical elements in the ECU are protected from high-voltage ESD.
In the electrical and electronic system, the electrical equipment is connected to a power supply circuit and the ground, and a high side circuit or a low side circuit are provided. In the high side circuit, electrical elements are arranged between the power supply circuit and the electrical equipment. In the low side circuit, electronic elements are arranged between the electrical equipment and the ground. Furthermore, the electrical and electronic system may have both the high side circuit and the low side circuit.
For example, in the case of an air-bag ECU for a vehicle, transistors are arranged in the high side circuit and the low side circuit. The control of electrical equipment (squib) for activating or inflating an air-bag is performed by controlling ON/OFF operations of the transistors, as disclosed in U.S. Pat. No. 6,465,907 (JP 2001-239916A). In this case, as shown in FIG. 4, capacitors 102 are provided between the electrical equipment 101 and both of the high side circuit and the low side circuit, respectively. Thus, both transistors 103 in the high side circuit and the low side circuit are protected.
Furthermore, one zener diode may be connected to a squib in parallel, and two zener diodes whose anodes are connected to the ground are provided to both of a high side circuit and a low side circuit in an air bag ECU, whereby transistors are protected from ESD. In this case, three zener diodes are used for one electrical equipment.
It is usual that plural kinds of electrical equipment are connected to the electrical and electronic system. For example, when activation of plural air-bags is controlled in the air-bag ECU, the electrical and electronic system has plural squibs, and high and low side circuits whose numbers correspond to the number of squibs are provided in the ECU.
Here, it is assumed that electronic elements are protected from ESD by using a capacitor. In this case, ESD flows through the capacitor to the ground at all times, and thus the capacitor is liable to be deteriorated. In connection with the deterioration of the capacitor, the insulating resistance value of the capacitor is lowered, so that the voltage of an electrical line 104 in FIG. 4 for connecting the electrical equipment and the electronic elements may vary and become unstable. Particularly when the electronic element is a transistor, the voltage variation may cause malfunction of the transistor. Furthermore, the deterioration of the capacitor lower the protection precision of the electronic elements.
Furthermore, in this case, it is required to provide capacitors 102 whose number correspond to the number of the high side circuit and the low side circuit and the plural kinds of electronic equipment 101. For example, when the air-bag ECU has twenty squibs, capacitors must be provided to both sides of each squib. Therefore, in this case, forty capacitors must be provided. That is, in the electrical and electronic system having the high side circuit and the low side circuits and the plural kinds of electrical equipment, the cost is greatly increased and reduction in size of the ECU is obstructed.
Still furthermore, with respect to ESD protection using only zener diodes, the zener diode directly leaks ESD to the ground, and thus the zener voltage may be varied in accordance with current flowing through the zener diode itself because of the characteristic of the zener diode. That is, when current caused by ESD flows into the zener diode itself, zener voltage is varied, the zener diode is deteriorated, etc., so that the ability of protecting the electronic elements is lowered. Furthermore, as in the case of the capacitor, it is required to provide zener diodes whose number correspond to the number of the high side circuits and the low side circuits and plural kinds of electrical equipment, so that the cost is greatly increased and reduction in size of the ECU is obstructed.
In addition, the high side circuit has less number of passages through which ESD leaks to the ground as compared with the low side circuit. Therefore, it is required to provide a more effective way to leak ESD to the ground in the high side circuit.